You get pregnant when you are ovulating or close to ovulation date. But for many it is hard to know when they ovulate. It's mostly a guessing game. Ovulation happens generally 14 days before your next period, so unless you are on a 28 day cycle every month it's hard to know when ovulation happens. You should keep in mind that sperm lasts 3-5 days in a woman, so there is a good 10 day period when you may get pregnant.

My aunt conceived the day after her period and her dr told her that she actually conceived 2 children the same way! It seems impossible when you read about the 'normal' monthly cycle but for a dr to get caught out it just goes to show that not everyone is the 'norm'!